Process for making durable rutile titanium dioxide pigment by vapor phase deposition of surface treatments

ABSTRACT

The present invention relates to a process for making durable titanium dioxide pigment by vapor phase deposition of surface treatments on the titanium dioxide particle surface by reacting titanium tetrachloride vapor, an oxygen containing gas and aluminum chloride in a plug flow reactor to form a product stream containing titanium dioxide particles; and introducing silicon tetrachloride into the reactor at a point down stream of the point where the titanium tetrachloride and oxygen were contacted and where at least 97% of the titanium tetrachloride has been converted to titanium dioxide or where the reaction temperature is no greater than about 1200° C., and preferably not more than about 1100° C.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to the chloride process for theproduction of titanium dioxide pigment. This invention provides a routeto a durable grade pigment, without the necessity of depositing surfacetreatments on the titanium dioxide particles by wet treatment.

[0002] Typically titanium dioxide particles, produced by either thechloride or the sulfate process, are processed in one or more wettreatment operations to deposit metal oxides on the surface of thepigment in order to optimize the pigment properties of dispersion,optical spacing or durability. Deposits of aluminum oxide orcombinations of aluminum oxide and silicon dioxide, used alone or incombination with other oxides, are typical constituents of commercialtitanium dioxide pigment. Such surface treatments are deposited throughprecipitation of the desired metal oxide in a wet chemical reaction.Thus, the base pigment, that is, the titanium dioxide particles producedat the exit point of the oxidizer in the chloride process or aftercalcination in the sulfate process, must be washed and processed throughone or more wet treatment steps. Wet treatment is then followed bywashing, drying and grinding to produce a product suitable for use infor example, exterior coatings and plastics products.

[0003] Processes to influence the titanium dioxide crystal formation inthe oxidizer of the chloride process were taught in British Patent689,123, and U.S. Pat. Nos. 3,856,929; 4,124,913; and 5,562,764.

[0004] U.S. Pat. No. 3,856,929 teaches that by oxidizing a mixed streamcontaining titanium tetrachloride, a silicon halide and a phosphorushalide, the resulting titanium dioxide product was at least 80% byweight anatase.

[0005] U.S. Pat. No. 4,124,913 teaches a process producing rutiletitanium dioxide particles at reduced levels of aluminum chlorideconcentration by oxidizing aluminum trichloride simultaneously with thetitanium chloride followed by addition of phosphorous trichloride. Thephosphorous trichloride is added at a point in the oxidizer where atleast 80% of the titanium tetrachloride has been converted to titaniumdioxide.

[0006] British Patent 689,123 teaches the oxidation of a mixture oftitanium tetrachloride, aluminum trichloride and silicon tetrachloridewhere the ratio of aluminum oxide to silicon dioxide formed in theoxidation is from 3:1 to 1:1 and where the temperature is maintained inthe range of 1000° C. to 1600° C. By this process it is claimed that 90%of the titanium dioxide formed is in the rutile crystal, and itsparticle size is about 0.5 microns or less.

[0007] U.S. Pat. No. 5,562,764 teaches a process for oxidizing a mixtureof titanium tetrachloride and aluminum trichloride followed by theaddition of silicon tetrachloride at a point down stream where thetemperature at the addition point is in the range of 1200 to 1600° C.The inventor in this patent wanted to enhance pigment gloss and carbonblack undertone (CBU) without producing a significant anatase componentin the pigment product. Although the product according to this patentcontained no more than 0.7 percent anatase, wet treatment was requiredto produce a durable and suitably dispersible pigment to meet industrystandards.

[0008] U.S. Pat. No. 3,219,468 discloses the addition of silicontetrachloride at a point removed from the addition point of the aluminumtrichloride in a fluidized bed oxidation of titanium tetrachloride. Thelater addition of the silicon tetrachloride results in the production ofa soft bed of titanium dioxide particles instead of a hard scale on thewalls of the fluidized bed reactor.

[0009] So-called vapor or dry process to deposit surface treatments onthe pigment in the oxidation step are taught in U.S. Pat. No. 4,050,951;PCT published patent application WO 96/36411; and European Patent 0 032426. In U.S. Pat. No. 4,050,951 post treatment hydrolysis is taught. Thedisadvantage in this system is that the treatment step is a separatestage in the overall process following oxidization that requires theseparation of base pigment from the oxidation product, then grindingfollowed by hydrolysis at temperatures lower than those temperaturespresent in the oxidizer.

[0010] PCT application WO 96/36441 teaches a vapor phase treatmentprocess requiring that the silicon tetrachloride addition must be madeat a temperature of more than 1300° C. This application further teachesthat the addition of metal halides can be made in any sequence and atany point in the reactor.

[0011] European Patent 0 032 426 teaches a post treatment of titaniumdioxide particles in a fluid bed reactor. This process requires anactivation step where the titanium dioxide particles are contacted withmetal chlorides followed by a hydrolysis to convert residual chloridesto oxides and oxide hydrates.

[0012] A common teaching in the art noted above is that the addition ofsilicon tetrachloride to the chlorination reaction is made at atemperature of at least 1200° C. and at a point relatively close to thepoint where the titanium tetrachloride was contacted by oxygen. Thecommon belief at the time of the present invention was made was that attemperatures 1200° C. or less the rate of silicon tetrachlorideconversion was so slow that pigment product would be contaminated byunreacted silicon tetrachloride as is taught in the article by D. R.Powers, “Kinetics of SiCl₄ Oxidation” published in J.Am. Ceram. Soc,vol. 61, No. 7-8, pp. 295-7 (1978). According to this understanding ofreaction kinetics, even in the presence of excess oxygen, addition ofsilicon tetrachloride at temperatures of less than about 1300° C. wouldresult in unreacted silicon tetrachloride in the product.

[0013] The present invention provides a process for the making of adurable grade pigment product in the oxidation unit of a chlorideprocess titanium dioxide plant by adding silicon tetrachloride late inthe reaction at a point where the reaction temperature is no greaterthan about 1200° C. and the base pigment is essentially formed. Theinventors of the present process wanted to provide a process to make adurable grade commercial product of acceptable gloss and CBU but withoutthe cost and additional processing required in the typical wet treatmentoperation.

BRIEF SUMMARY OF THE INVENTION

[0014] The present invention provides a process for making durabletitanium dioxide pigment by vapor phase deposition of surface treatmentson the titanium dioxide pigment particle surface, the process comprisingthe steps of:

[0015] (a) reacting titanium tetrachloride vapor and aluminum chlorideand at least a stoichiometric amount of oxygen in a plug flow reactor toform a product stream containing titanium dioxide particles; and

[0016] (b) introducing silicon tetrachloride into the reactor at one ormore points downstream of the point where the titanium tetrachloride andoxygen were contacted and where at least 97% of the titaniumtetrachloride has been converted to titanium dioxide.

[0017] Before the introduction of the silicon tetrachloride, it is morepreferred that at least 98% of the titanium tetrachloride has beenconverted to titanium dioxide, and most preferred that at least 99% ofthe titanium tetrachloride has been converted.

[0018] It is also preferred that the silicon tetrachloride is introducedin an amount sufficient to provide a silicon dioxide content of asurface treated titanium dioxide pigment of about at least 1.2% byweight, and that the aluminum trichloride is added in an amountsufficient to provide an aluminum oxide content of a surface treatedpigment of at least about 1% by weight.

[0019] Steam or oxygen may be introduced at a point downstream of thepoint addition of the silicon tetrachloride, or steam or oxygen may beintroduced along with the silicon tetrachloride.

[0020] The present invention also provides a durable titanium dioxidepigment wherein at least 95% of the pigment particles are completelycovered by a layer formed from a mixture of amorphous aluminum oxide andamorphous silicon dioxide, the pigment produced by:

[0021] (a) reacting titanium tetrachloride vapor and aluminum chlorideand at least a stoichiometric amount of oxygen in a plug flow reactor toform a product stream containing titanium dioxide particles; and

[0022] (b) introducing silicon tetrachloride into the reactor at one ormore points downstream of the point where the titanium tetrachloride andoxygen were contacted and where at least 97% of the titaniumtetrachloride has been converted to titanium dioxide.

[0023] The preferred composition of the pigment of the present inventionis that the concentration of silicon dioxide is at least about 1.2% ofthe total weight of the pigment and the concentration of the aluminumoxide is at least about 1% of the total weight of the pigment.

[0024] The present invention provides durable titanium dioxide pigmentparticles having a surface treatment layer comprising aluminum oxide andsilicon dioxide wherein at least 85% of the pigment particles arecompletely covered by a uniform layer formed from a mixture of amorphousaluminum oxide and amorphous silicon dioxide and wherein the pigmentparticles are free of debris.

[0025] The present invention also provides a method to determine a pointof introduction of silicon tetrachloride into a plug flow reactor forthe oxidation of a mixture of titanium tetrachloride and aluminumtrichloride in a gas containing at least the stoichiometric amount ofoxygen to produce titanium dioxide particle having a thin, uniform andcomplete layer of surface oxides comprising a mixture of amorphousaluminum oxide and silicon dioxide, the steps of the process comprising;

[0026] (a) determining the temperature in the reactor where not morethan about 3% of the titanium tetrachloride remains unreacted using$K = \frac{\left\lbrack {{2\left( {{100\%} - u_{TiCl4}} \right)} + {\varphi \times 100\%}} \right\rbrack^{2}}{u_{TiCl4}\left( {\beta + u_{TiCl4}} \right)}$and $T < {\frac{20733}{{\ln \quad K} + 6.391} - 273.15}$ where

[0027] μ_(TiCl4)=unreacted TiCl4(%)

[0028] β=O2(%) in excess of the stoichiometric amount

[0029] φ=feed Cl2 mole ratio (mol/mol TiCl4), and

[0030] T=temperature (C);

[0031] and

[0032] (b) introducing the silicon tetrachloride into the reactor wherethe temperature is equal to or less than the temperature calculated instep a.

[0033] In addition, the present invention includes a process for makingdurable titanium dioxide pigment by vapor phase deposition of surfacetreatments on the titanium dioxide pigment particle surface, the processcomprising the steps of:

[0034] (a) reacting titanium tetrachloride vapor, an oxygen containinggas and aluminum chloride in a plug flow reactor to form a productstream containing titanium dioxide particles; and

[0035] (b) introducing silicon tetrachloride into the reactor at one ormore points downstream of the point where the titanium tetrachloride andoxygen were contacted and where the reaction temperature is no greaterthan about 1200° C. and more preferred no greater than 1100° C.

BRIEF DESCRIPTION OF THE DRAWING(S)

[0036]FIG. 1a and 1 b show the pigment particles of the presentinvention. FIG. 1a is a micrograph of these particles. FIG. 1b shows aHREM atomic resolution micrograph of the pigment of the presentinvention.

[0037]FIG. 2 shows a micrograph of a typical wet treated durable gradetitanium dioxide commercial pigment.

DETAILED DESCRIPTION OF THE INVENTION

[0038] The present invention provides a process for making durabletitanium dioxide pigment by vapor phase deposition of surface treatmentson the titanium dioxide particle surface, the process comprising thesteps of:

[0039] (a) reacting titanium tetrachloride vapor and aluminum chlorideand at least a stoichiometric amount of oxygen in a plug flow reactor toform a product stream containing titanium dioxide particles; and

[0040] (b) introducing silicon tetrachloride into the reactor at one ormore points downstream of the point where the titanium tetrachloride andoxygen were contacted and where at least 97% of the titaniumtetrachloride has been converted (3% unreacted titanium tetrachloride)to titanium dioxide.

[0041] The term durable as used herein means a pigment suitable forexterior architectural coatings and automotive refinish or colorcoat/clear coat OEM finishes. Generally such pigments are characterizedin that no more than about 25% of the pigment dissolves in sulfuric acidin the acid solubility test as described below, and that silicon dioxiderepresents at least 1.4 to 2% of the total weight of the pigment.

[0042] The composition of the oxide treatment deposited by the processof the present invention is a mixture of amorphous aluminum oxide andamorphous silicon dioxide. The thickness of the treatment layerdeposited in the present invention is not more than about 4 nm. Thepigment is more than 99% rutile.

[0043] The uniformity of the surface treatment according to the presentinvention may be seen in FIGS. 1a and 1 b. FIG. 1a shows aphotomicrograph of pigment particles of the present invention. Thetreatment on the surface of these particles can be seen to be completeand uniform. It is believed that the uniformity and the completeness ofthe surface treatment layer in the present pigments results in acidsolubilities of less that 25% even at silica concentrations of about 1%by weight of the total pigment.

[0044]FIG. 1b shows an atomic resolution HREM of a typical particleproduct of the present invention. In this FIG. the region “P” indicatesthe titanium dioxide particle, and the layer “C” is the treatment layerof amorphous oxides of aluminum and silicon. The uniformity and fullextent of coverage of the layer is readily visible.

[0045] Analysis of samplings of 1000 particles as described below wasused to determine the fraction of particles treated having full,complete surface coverage. When least 85% of the particles had full,complete surface coverage (FIGS. 1a, 1 b), the acid solubility of theseparticles was equal to that of durable, commercial grade products.Experience has shown that acid solubility of 25% or less correlates withoutdoor exposure required for commercial grade durable pigments. Thus,acid solubility serves as an accelerated durability test.

[0046] Full, complete coverage of the particles means that the entiresurface of the titanium dioxide particle is covered with the layer ofsurface treatment. The product of the present invention is characterizedby the fact that at least 85% of the particles are fully and completelycovered by a layer of surface treatment. This layer is thin and uniform.The thickness of the layer is about 1 to 4 nm for particles that areabout 1% by weight aluminum oxide and 1.2% by weight silicon dioxide.Higher concentrations of the surface treatment are expected to producethicker layers, but at equal uniformity. Microscopic analysis of theproduct of the Example has shown that about 80% or more of the pigmentparticles of the present invention have a treatment layer thickness of 1to 2.5 nm, while in less than about 5% of the pigment particles, thetreatment layer is about 4 nm thick.

[0047] The inventors believe that it is the completeness of surfacecoverage resulting from the process of the present invention that mayprovide pigment durability meeting industry standards at treatmentlevels of about 2 to 3% of the total pigment weight compared totreatment levels of about 6% required for typical architectural durablegrade titanium dioxide pigment. In each of the treatment levels notedimmediately above, the weight percent shown is the combinedconcentrations of aluminum oxide and silicon dioxide.

[0048] In comparison to the present invention, wet treatment processesdeposit silicon dioxide and aluminum oxide on to the surface of thepigment particle by precipitation. Wet treatment processes typicallyproduce silica debris, crystalline aluminum oxide, and irregularparticle surfaces such as shown in FIG. 2. Crystalline oxides typical ofwet treatments are not found in the product of the present invention.

[0049] Elimination of wet treatment offers an advantage in the overalltitanium dioxide manufacturing process in reducing processing steps. Newtreatment compositions offer the potential to produce pigments havingimproved processing characteristics and properties.

[0050] In contrast to pigments produced by wet treatment processes, thepigment of the present invention is free of debris. This lack of debrismay contribute to improved dispersion and improved performance in(coatings and plastics. The presence of debris may be seen in FIG. 2.

[0051] Although pigment durability can be achieved at levels of silicondioxide of about 1% by weight of the pigment, higher levels of silicondioxide and of aluminum oxide may be deposited on the surface of apigment of the present process. Also other oxides may be deposited usingthe present process, and the pigment of the present invention may betreated with organic treatments as is known by one skilled in this art.Although a durable pigment is produced by the process of the presentinvention when at least 85% of the particles have a full, completesurface coverage, it is preferred that at least 95% of particles havingfull, complete surface coverage, and a fraction of about 98% or more iseven more preferred.

[0052] In the present process, titanium tetrachloride is preheated to atemperature of from about 300 to 650° C. and mixed with aluminumtrichloride forming a chloride mix which is fed into a pre-heated streamof oxygen. This chloride mix may contain other metal compounds, exceptsilicon tetrachloride, used in the chloride pigment manufactureincluding compounds of boron, phosphorous, zirconium, and others. Theintroduction of phosphorous compounds into the oxidizer is generallypositioned to control corrosion and may be at some point down stream ofthe point where titanium tetrachloride and aluminum trichloride areintroduced into the reactor.

[0053] According to the present invention, it is essential that thealuminum trichloride be added in advance of and in a location farremoved from the point where silicon tetrachloride is introduced intothe reactor. Thus, the preferred location for the addition of thealuminum trichloride is in a mixture with the titanium tetrachloride.

[0054] In the process of the present invention, oxygen is present as aninitial reactant and may also be added with the addition of the silicontetrachloride. Although it is preferred to run the present process withthe oxygen in excess of the amount required to oxidize the chloride mix,the process may be operated with the concentration equal to or less thanthe stoichiometric amount.

[0055] The addition of silicon tetrachloride according to the presentinvention is made when the conversion of titanium tetrachloride totitanium dioxide is nearly complete. For example, at least 97% of thetitanium tetrachloride has been converted to titanium dioxide. That is,the point where not more than 3% of the titanium tetrachloride remainsunreacted. From their work the inventors have found that the point inthe reactor where about 3% of the titanium tetrachloride is unreacted,the fraction of particles having full, complete coverage by the surfacetreatment is about 85%. At the point in the reactor where about 2% ofthe titanium tetrachloride is unreacted, the fraction of particleshaving full, complete coverage by the surface treatment is about 95%. Atthe point in the reactor where about 1% of the titanium tetrachloride isunreacted, the fraction of particles having full, complete coverage bythe surface treatment is more than about 98%. The corresponding amountof titanium tetrachloride converted to titanium dioxide at these pointsis at least 97%, at least 98% and at least 99%, respectively.

[0056] When the present process is run as preferred, with at least thestoichiometric amount of oxygen, the addition points for silicontetrachloride may be calculated by the following equations:$K = \frac{\left\lbrack {{2\left( {{100\%} - u_{TiCl4}} \right)} + {\varphi \times 100\%}} \right\rbrack^{2}}{u_{TiCl4}\left( {\beta + u_{TiCl4}} \right)}$and $T < {\frac{20733}{{\ln \quad K} + 6.391} - 273.15}$ where

[0057] μ_(TiCl4)=unreacted TiCl4(%)

[0058] β=excess O2(%)

[0059] φ=feed Cl2 mole ratio (mol/mol TiCl4), and

[0060] T=temperature (C)

[0061] K is the equilibrium constant for the reaction of the presentprocess:

TiCl₄+O₂→TiO₂+2Cl₂;

[0062] Using this equation, one may calculate the point where thesilicon tetrachloride is first introduced from the feeds going into thereactor. Excess oxygen, β, is the oxygen in excess of that required toconvert the mixture of titanium tetrachloride and aluminum trichloridefed into the reactor to their respective oxides (the stoichiometricamount). The feed chlorine mole ratio, φ, is the ratio of the moles ofchlorine fed divided by the moles of titanium tetrachloride fed to thereactor over a fixed period of time, for example, per hour. The percentunreacted titanium tetrachloride, μ_(TiCl4), is not more than 3% as isrequired by the present invention. Using the calculated equilibriumconstant, K, one can then solve for the temperature at the point wheresilicon tetrachloride is first introduced according to the presentinvention. The point in the reactor where this introduction is madeaccording to the present invention may be determined using thetemperature profile of the particular reactor.

[0063] This calculation is independent of reactor size and pressure andrequires only knowledge of the feed composition (oxygen, chlorine andtitanium tetrachloride in moles per hour) and the temperature profilefor the reactor. Temperature profiles for a given reactor may bedetermined from well-known thermodynamic and heat transfer principles.

[0064] This method of calculating the addition points provide someflexiblilty, based on the feed mix that may be of importance indesigning product features to serve a particular pigment end useapplication.

[0065] The present process for making durable titanium dioxide pigmentby vapor phase deposition of surface treatments on the titanium dioxidepigment particle surface may also be operated with a mixture of titaniumtetrachloride and aluminum trichloride where the oxygen may be presentin an amount less than the stoichiometric amount. In this case theprocess comprising the steps of:

[0066] (a) reacting titanium tetrachloride vapor, an oxygen containinggas and aluminum chloride in a plug flow reactor to form a productstream containing titanium dioxide particles; and

[0067] (b) introducing silicon tetrachloride into the reactor at one ormore points downstream of the point where the titanium tetrachloride andoxygen were contacted and where the reaction temperature is no greaterthan about 1200° C.

[0068] In this case one would use the reactor temperature profile tolocate a point where the reaction temperature is no greater than about1200° C., and preferably no greater than 1100° C. The addition ofsilicon tetrachloride would be made at this point or a point down streamof this calculated location. The use of the temperature profile and therequirement that the addition of silicon tetrachloride be made at alocation where the reaction temperature is less than about 1200° C. isuseful in cases where oxygen is present in excess, just equal to or lessthan the stoichiometric amount needed to oxidize the chloride mix.

Test Methods

[0069] Acid Solubility is determined as the amount of pigment thatdissolves in hot concentrated sulfuric acid.

[0070] A small sample of pigment was placed in hot sulfuric acid (about175° C.) and digested for an hour. The sample was then diluted with ameasured amount of water and all particulate material was filtered out.A measured sample of the filtrate was then placed in a volumetric flask.Hydrogen peroxide was added to the flask to ensure all the titanium ionswere in the proper oxidation state for their concentration to bedetermined spectrophotometrically at 400 nm. The flask was then filledto volume with 10% sulfuric acid. The absorbance was measured vs. ablank containing the same amount of hydrogen peroxide as was added tothe sample in 10% sulfuric acid. The percent of titanium dioxide wasread from a calibration curve prepared from known standards.

[0071] High Resolution Electron Microscopy Procedures:

[0072] A combination of high resolution transmission EM (HREM) withatomic resolution and high resolution low voltage scanning EM (LVSEM)was used to determine the microstructure, morphology, treatment layerthickness, uniformity and chemical composition.

[0073] Microstructure and high precision chemical compositional analyseson a (sub)nanometer scale were carried out by HREM and the associatedelectron stimulated energy dispersive X-ray compositional spectroscopy(EDX), respectively. A Philips CM200 field emission gun HREM/STEM,Philips CM20 HREM and a modified Philips CM30 environmental-HREMinstruments were used in the investigations, with an acceleratingvoltage of 200 kV (ref: P.L. Gai, DuPont: published in AdvancedMaterials, Vol. 10, p. 1259, 1998). All the EMs were equipped with X-rayspectrometers to analyze chemical composition.

[0074] The extent of treatment and treatment layer coverage observationswere made on all sides (including top and bottom surfaces) of theparticles using standard sample tilting methods. For HREM, the pigmentcrystals were oriented so that the desired crystal axes (e.g. <010>)were exactly parallel to the electron beam. Primary magnifications were100,000 to 750,000.

[0075] A minimum sampling of 1000 particles having variable particlesize and dimensionality was studied to represent an accurate measure ofthe fraction particles treated and the extent of the treatment surfacecoverage. HREM at atomic resolution was used to determine monolayercoatings as well as nanometers-scale coatings. Observations ofirregularity in treatment layers of partially coated and fully coatedparticles were carried out. Histograms were prepared according tostandard statistical methods were used to determine the fraction ofparticles where the treatment layer was full and complete at treatmentlayer thickness.

EXAMPLE

[0076] Titanium tetrachloride was pre-mixed with aluminum trichloride(chloride mix) and fed to the oxidation reactor. The amount of aluminumtrichloride in the mixture was sufficient to provide about 1 wt %aluminum oxide based on total solids formed in the oxidation reactor.

[0077] The chloride mix was evaporated and pre-heated to about 450° C.and introduced into the reaction zone. Simultaneous with theintroduction of the chloride mix, pre-heated oxygen (where the totalexcess oxygen was about 14 mole %) was continually introduced through aseparate inlet adjacent to the chloride mix inlet. Trace amounts of KCldissolved in water was added to the oxygen stream as disclosed inBritish Patent 922,671 and U.S. Pat. No. 3,202,866. The Cl₂/TiCl₄ moleratio in the feed was 1:1.

[0078] Reaction temperature where the chloride mix contacted the oxygenwas about 1550° C. Silicon tetrachloride was added as a dispersed liquiddown stream from where the chlorides mix and the oxygen streams wereinitially contacted at the point where approximately 1% of the titaniumtetrachloride remained unconverted. That is, more than 99% of thetitanium tetrachloride had been converted to titanium dioxide. Thesilicon tetrachloride was added in an amount sufficient to yield apigment having 1.2% of its total weight as silicon dioxide. Thetemperature at this point of addition was estimated to be approximately1100° C.

[0079] In the prior art, addition of the silicon tetrachloride at such alow temperature was considered impossible because of the concern thatsilicon tetrachloride would not have sufficient temperature to beconverted completely to the oxide. Although known reaction modelspredicted incomplete reaction of the silicon tetrachloride and oxygen atthe 1100° C.-reaction temperature, there was no unreacted silicontetrachloride in the product exiting the reactor.

[0080] The product pigment produced had the following properties. Thefraction of the particles that were covered by the treatment layer was97%. The treatment layer was uniformly deposited as is shown in FIGS. 1aand 1 b. The acid solubility of this pigment was about 21%. Acidsolubility of a typical wet treated durable grade is about 25%.

[0081] A second feature of the pigment produced was the low moisturecontent as measured using standard TGA methods. Losses of weight at 300°C. for the present product was about 0.6% and at 600° C. about 0.9%compared to commercial product weight losses of 0.9% and 1.6%,respectively. Such low moisture content may be preferred for a pigmentin plastic extrusion films.

[0082] The product pigment composition was 1.2% silicon dioxide and 1%aluminum oxide.

Comparative Example

[0083] A control test was made according to U.S. Pat. No. 5,562,764 toGonzalez. The reaction conditions were the same as in the test above,except the addition of silicon tetrachloride was made at a point near 5feet downstream from the point where the oxygen and chlorides mixstreams were initially contacted and at a temperature of 1400 to 1500°C. At this point more than 5% of the titanium tetrachloride wasunreacted. The fraction of the particles that were covered by atreatment layer was only 16%. That is, about 84% of the pigmentparticles had surfaces that were not covered. The acid solubility ofthis pigment was about 35%.

[0084] In Gonzalez the addition of silicon tetrachloride was made toinfluence the crystal form and pigment carbon black undertone of thepigment produced. The addition of silicon tetrachloride in the presentinvention is made at a temperature less than 1200 C. and preferred to beno greater than about 1100° C. At such temperatures the particles areessentially formed, and the silicon tetrachloride addition does notinfluence either crystal phase or pigment carbon black undertone.

What is claimed is:
 1. A process for making durable titanium dioxidepigment by vapor phase deposition of surface treatments on the titaniumdioxide pigment particle surface, the process comprising the steps of:(a) reacting titanium tetrachloride vapor and aluminum chloride and atleast a stoichiometric amount of oxygen in a plug flow reactor to form aproduct stream containing titanium dioxide particles; and (b)introducing silicon tetrachloride into the reactor at one or more pointsdownstream of the point where the titanium tetrachloride and oxygen werecontacted and where at least 97% of the titanium tetrachloride has beenconverted to titanium dioxide.
 2. The process of claim 1 wherein thesilicon tetrachloride is introduced at a point where at least 98% of thetitanium tetrachloride has been converted to titanium dioxide.
 3. Theprocess of claim 1 wherein the silicon tetrachloride is introduced at apoint where at least 99% of the titanium tetrachloride has beenconverted to titanium dioxide.
 4. The process of claim 1 wherein thesilicon tetrachloride is introduced in an amount sufficient to provide asilica content of a surface treated titanium dioxide pigment of about atleast 1.2% by weight and the aluminum trichloride is added in an amountsufficient to provide an aluminum oxide content of a surface treatedpigment of at least about 1% by weight.
 5. The process of claim 1wherein steam or oxygen is introduced at a point downstream of the pointof introduction of silicon tetrachloride or wherein steam or oxygen areintroduced along with the silicon tetrachloride.
 6. A durable titaniumdioxide pigment wherein at least 85% of the pigment particles arecompletely covered by a uniform layer formed from a mixture of amorphousaluminum oxide and amorphous silicon dioxide, the pigment produced by:(a) reacting titanium tetrachloride vapor and aluminum chloride and atleast a stoichiometric amount of oxygen in a plug flow reactor to form aproduct stream containing titanium dioxide particles; and (b)introducing silicon tetrachloride into the reactor at one or more pointsdownstream of the point where the titanium tetrachloride and oxygen werecontacted and where at least 97% of the titanium tetrachloride has beenconverted to titanium dioxide.
 7. The pigment of claim 6 wherein thepercent of silicon dioxide is at least about 1.2% of the total weight ofthe pigment and the aluminum oxide is at least about 1% of the totalweight of the pigment.
 8. Durable titanium dioxide pigment particleshaving a surface treatment layer comprising aluminum oxide and silicondioxide wherein the at least 85% of the pigment particles are completelycovered by a uniform layer formed from a mixture of amorphous aluminumoxide and amorphous silicon dioxide and wherein the pigment particlesare free of debris.
 9. The titanium dioxide pigment of claim 8 wherein95% or more of the pigment particles are completely covered by a uniformlayer formed from a mixture of amorphous aluminum oxide and amorphoussilicon dioxide.
 10. A method to determine a point of introduction ofsilicon tetrachloride into a plug flow reactor for the oxidation of amixture of titanium tetrachloride and aluminum trichloride in a gascontaining at least a stoichiometric amount of oxygen to producetitanium dioxide particle having a thin, uniform and complete layer ofsurface oxides comprising a mixture of amorphous aluminum oxide andsilicon dioxide, the steps of the process comprising; (a) determiningthe temperature in the reactor where not more than 3% of the titaniumtetrachloride remains unreacted using$K = \frac{\left\lbrack {{2\left( {{100\%} - u_{TiCl4}} \right)} + {\varphi \times 100\%}} \right\rbrack^{2}}{u_{TiCl4}\left( {\beta + u_{TiCl4}} \right)}$and $T < {\frac{20733}{{\ln \quad K} + 6.391} - 273.15}$ where

μ_(TiCl4)=unreacted TiCl4(%) β=O2(%) in excess of the stiochiometricamount φ=feed Cl2 mole ratio (mol/mol TiCl4), and T=temperature (C); and(b) introducing the silicon tetrachloride into the reactor where thetemperature is equal to or less than the temperature calculated in step(a).
 11. A process for making durable titanium dioxide pigment by vaporphase deposition of surface treatments on the titanium dioxide pigmentparticle surface, the process comprising the steps of: (a) reactingtitanium tetrachloride vapor, an oxygen containing gas and aluminumchloride in a plug flow reactor to form a product stream containingtitanium dioxide particles; and (b) introducing silicon tetrachlorideinto the reactor at one or more points downstream of the point where thetitanium tetrachloride and oxygen were contacted and where the reactiontemperature is no greater than about 1200° C.